Depends on which state you're in. most start at 16. You can get a cosigned loan..a credit card..bank accts with CD's that gain interest and provide credit history. Ask an adult where you live or better yet, ask an economics teacher at school or go with your parents when they file their yrly taxes shortly.
basically 18+ when you get a loan on something or buy a car even cheap one through a bank and pay it back. when you can do stuff like that legally. I bought a $1200 car $1000 was on a loan paid it off in less then a year and now I got decently good credit off that one loan. I was 20 when I did that. would of done sooner but was not in books of life at time.